1. Field of the Invention
The present invention relates to a method and an apparatus for detecting a decrease in a tire air pressure and a program for determining a decrease in a tire air pressure.
2. Description of the Related Art
Conventionally, an apparatus for detecting a decrease in a tire air pressure uses a principle according to which a tire having a decreased pressure shows a reduced outer diameter (tire dynamic loaded radius) compared to that of tires having a normal air pressure and thus shows an increased angular velocity (wheel speed) compared to that of other normal tires. In the case of a method of detecting a decrease in an air pressure based on a relative difference in a tire wheel speed for example, a judgment value DEL obtained by DEL={(V1+V4)/2−(V2+V3)/2}/{(V1+V2+V3+V4)/4}×100(%) is used. When an absolute value of this DEL exceeds an alarming threshold value, an alarm is raised (see Japanese Unexamined Patent Publication No. 305011/1988 for example). The reference numerals V1 to V4 represent the wheel speeds of a left front wheel tire, a right front wheel tire, a left rear wheel tire, and a right rear wheel tire, respectively.
When the judgment value DEL as described above is used to judge a decreased pressure of a tire, a tire slip rate caused during driving changes due to an unbalanced load for example. In this case, the DEL, which is calculated based on a relative difference in the wheel speed, cannot exclude the influence by the slip rate, consequently causing a wrong judgment of a decreased pressure in spite of a normal pressure.
To solve this, a method of detecting a decrease in a tire air pressure has been proposed according to which three judgment values DEL1, DEL2, and DEL3 can be used to raise a decreased pressure alarm and to identify a position of a wheel having a decreased pressure while avoiding a false judgment (see Japanese Unexamined Patent Publication No. 2005-53263). The judgment value DEL1 is obtained by dividing, by a predetermined average wheel speed, a difference of the sum of the wheel rotation information between two pairs of wheel tires on diagonal lines. The judgment value DEL2 is obtained by dividing, by a predetermined average wheel speed, a difference between a sum of the wheel rotation information for front wheel tires and a sum of the wheel rotation information for rear wheel tires. The judgment value DEL3 is obtained by dividing, by a predetermined average wheel speed, a difference between a sum of the wheel rotation information for right wheel tires and a sum of the wheel rotation information for left wheel tires.
Among the three judgment values, the judgment value DEL2 is generally considered to be changeable depending on driving torque or a vehicle velocity. In order to judge whether a tire has a decreased pressure or not, it is required to perform initialization when a regular internal pressure is maintained to calculate an initial value of the judgment value DEL2 at the regular internal pressure. When the initial value of DEL2 is learned as a factor depending on driving torque for example, changes of DEL2 depending on the driving torque changing momentarily are stored and DEL2 is represented as a function of the driving torque to calculate an approximate expression showing the correlation between the DEL2 and the driving torque. In the case of a two-wheel driving vehicle, the relation between the former and the later as shown in FIG. 5 for example where the horizontal axis represents a driving torque and the vertical axis represents DEL2.
As shown in FIG. 5, the relation between the driving torque and DEL2 can be approximated by a linear function. Thus, A and B satisfying DEL2=A×driving torque+B are calculated during initialization. Based on this, an index for distinguishing a decreased pressure status from a regular internal pressure status is obtained. FIG. 6 illustrates the relation between the driving torque and DEL2 in the regular internal pressure status and the relation between the driving torque and DEL2 in the decreased pressure status. Assuming that a driving torque at a certain time t is T(t) for example, DEL2 at a regular internal pressure when the driving torque is T(t) is DEL2=A×T(t)+B. DEL2 at this time t can be calculated based on the wheel rotation information obtained from the respective wheel tires. By comparing this calculated DEL2 with DEL2 at the regular internal pressure, whether there is a tire having a decreased pressure or not can be determined.
In the case of a four-wheel driving vehicle, when driving torque is evenly distributed to the front and rear axes to establish equidistribution (50%:50%), DEL2 also has a substantially-constant value regardless of the magnitude of the driving torque as shown in FIG. 7 for example. Thus, this DEL 2 can be used as a reference value. By comparing this reference value with DEL2 calculated during the vehicle running, whether there is a tire having a decreased pressure or not can be determined. FIG. 8 illustrates a difference between DEL2 when a regular internal pressure is maintained and DEL2 when a decreased pressure is caused.
However, DEL2 depends on a driving torque or a vehicle velocity only in the case of a two-wheel driving vehicle. In the case of a four-wheel driving vehicle, DEL2 converges to a constant value only when the distribution of the front and rear driving torque is even to establish equidistribution (50%:50%). When the distribution of the front and rear driving torque of a four-wheel driving vehicle is variable, DEL2 does not converge to a constant value as shown in FIG. 9 and its correlation with the sum of the front and rear driving torque cannot always be found.
In the case of a vehicle that does not have equidistribution of the front and rear driving torque, DEL2 does not have a fixed relation to the sum (total value) of the front and rear driving torque. Thus, it is difficult to use this value of DEL2 to accurately distinguish a decreased pressure status from a regular internal pressure status.